This invention relates to method and apparatus for measuring a diameter of a single crystal being grown according to the Czochralski method, and more particularly, to method and apparatus for measuring a diameter of a single crystal, which enable one to accurately measure the diameter of a portion being grown in case where the single crystal, which is pulled up for its growth, is oscillated.
Many methods of preparing a single crystal are known, among which the Czochralski method has been widely applied to as a technique capable of industrial mass production with respect to the growth of a silicon single crystal.
FIG. 12 is a view showing an arrangement of silicon growing apparatus using the Czochralski method. The single crystal is grown in a container in a chamber 7 wherein a crucible 1 is place at the central portion thereof. A heater 2 is provided to surround the crucible 7 therewith, and a melt of a crystalline starting material, i.e., melt 3 of polysilicon, heated by means of the heater is accommodated in the crucible 1. A seed crystal 5 attached at the tip of a wire 4 is brought into contact with the surface of the melt 3 at the lower end thereof. This seed crystal 5 is pulled upwardly, whereupon a single crystal 6 grows at the lower end of the seed crystal as a result of the solidification of the melt. The thus grown single crystal 6 consists of a necking portion 6a, in which its diameter is reduced in order to exclude dislocation from the crystal surface, a shoulder portion 6b, in which its diameter gradually increases from the necking portion 6a, and a body portion 6c utilized as a wafer after growth.
In recent years, there is a great demand for a large diameter of a pulled single crystal, with the result that a single crystal being grown becomes great in weight. To this end, it becomes necessary to reduce the diameter of the single crystal at the necking portion thereof to an extent necessary for making a dislocation-free crystal. Moreover, the diameter should be one that is larger than a given dimension sufficient to tolerate the heavy weight of the single crystal. Accordingly, for the growth of a silicon single crystal, it is important to accurately measure and appropriately control the diameter of a single crystal at its necking portion.
From this point of view, there have been made proposals on the measurement of the diameter of a single crystal using (a) a one-dimensional CCD camera and (b) a two-dimensional CCD camera.
FIGS. 2 to 4 attached herewith are, respectively, views illustrating the manner of measuring the diameter according to conventional CCD cameras. More particularly, FIG. 2 shows the relation between the scanning line of a one-dimensional CCD camera and the single crystal growth portion, and FIG. 3 is a view illustrating how to determine the diameter of the single crystal growth portion from a luminance distribution image produced around the single crystal on the scanning line. FIG. 4 shows the relation between a plurality of scanning lines and the single crystal growth portion in case where a two-dimensional CCD camera is used.
(a) In order to measure the diameter of a single crystal by use of CCD cameras, usual practice is to observe a luminance distribution caused by the fusion ring of the single crystal growth portion via a window provided at the upper portion of a chamber by means of a CCD camera, which is provided at the outside of the chamber of a growing device. Where a one-dimensional CCD camera is used for the measurement, the scanning line G, shown in FIG. 2, should be so set as to go across the center of the diameter of a single crystal. The luminance distribution is measured under such conditions as mentioned above, and the diameter of the single crystal growth portion can be exactly obtained from the distance, db, between the luminance peaks of the fusion ring in the proximity of the single crystal growth portion. In general, where the scanning line crosses the center of the diameter of the single crystal, the distance, db, between the luminance peaks is measured to be slightly larger than the actual diameter of the single crystal.
At that time, the diameter at the necking portion of the single crystal is fine, and the single crystal being pulled by means of a wire is swung or oscillated during the course of the growth, so that it is difficult to scan the single crystal across the center of the diameter thereof by use of a one-dimensional CCD camera. To avoid this, there has been proposed a device for measuring a diameter which includes means for reciprocating a scanning line more quickly than a swinging or oscillating speed of a single crystal, and a detecting unit for selecting an image, wherein the scanning line is in coincident with the center of the diameter, from two-dimensional images obtained by reciprocating movement of the scanning lines and outputting a value of the diameter of the single crystal (see Japanese Laid-open Patent Application No. 62-138387).
This diameter-measuring device is not practically suited as a growing device because a mechanism of reciprocating the scanning line at high speed becomes complicated, and stable measurement over a long time is difficult to continue. Where a single crystal with a diameter of 8 inches (200 mm) at a body portion thereof is grown, for example, the diameter at the necking portion of the single crystal becomes as fine as around 5 mm. In addition, the oscillation of the single crystal may reach 2 to 3 mm in radius. In this case, the proposed device is not reliable because accuracy (with an error of 10% or below) required for general purposes cannot be obtained.
The required accuracy (with an error at 10%) is decided from the diameter of a necking portion necessary for dislocation-free crystallization (6 mm or below), the diameter, which is tolerable with the weight of a single crystal, (4 mm or over when the weight is at 150 kg), and the accuracy (xc2x120%), which is ensured by the control of the diameter of a single crystal.
(b) On the other hand, where the diameter of a single crystal is measured by use of a two-dimensional CCD camera, a single crystal growth portion is instantaneously scanned with a number of scanning lines G as shown in FIG. 4, thereby obtaining two-dimensional image information. In this case, if the single crystal is being swung or oscillated, the number of scanning lines contain a scanning line which goes across the center of the diameter of the single crystal. The diameters of the respective single crystal growth portions can be determined from the image signals of measured luminance distribution, among which the greatest value of the diameter is taken as a diameter of the growth portion. In this way, the diameter of the single crystal can be measured relatively accurately. Accordingly, there has been proposed a device of measuring a diameter using a two-dimensional CCD camera (see Japanese Laid-open Patent Application Nos. 62-87482 and 04-86509).
However, the resolving ability of the image with the two-dimensional CCD camera is much lower than that made by a one-dimensional CCD camera. Accordingly, when using the above type of measuring device for the measurement of a diameter at the necking portion, a high image accuracy cannot be expected, so that the diameter is measured while enlarging the image by use of a zooming device. On the other hand, as the diameter increases in the course of the growth of the shoulder and the following of the single crystal, a wide-range measuring field of view becomes necessary. This is beyond the measurement with a two-dimensional CCD camera, and it is necessary to continue measurement while switching over to a one-dimensional CCD camera. This means that the proposed diameter-measuring device includes, in combination, a measurement with a two-dimensional CCD camera using a zooming device and a measurement with a one-dimensional CCD camera. Thus, there arises the problem that the arrangement of the measuring device becomes complicated and large in scale, thus being not suited for practical applications.
Since there is a demand not only for a large diameter and a heavy weight of a pulling single crystal, but also for a high growth efficiency of the single crystal, the control of the diameter of the single crystal has to be performed automatically without resorting to hands (not manually). In order to meet the demand, it is premised on the assumption that the diameter is accurately measured over the overall length of a single crystal including a necking portion thereof.
Accordingly, it is an object of the invention to provide method and apparatus for measuring a diameter of a single crystal which can overcome the problems of the prior art techniques and wherein a one-dimensional CCD camera is used whereby one is enabled to accurately measure the diameter of not only a necking portion, but also growth portions including a shoulder portion and the following in the course of the growth of a single crystal.
In the growth of a single crystal according to the Czochralski method, it is difficult to avoid oscillation involved in the single crystal pulled by means of a wire. Hence, we drew attention to the fact that the oscillation has a given cycle on the assumption that a single crystal being grown is oscillated or swung, thus arriving at the completion of the invention. The subject matter of the invention resides in a method for measuring a diameter of a single crystal described under (1) below and also in an apparatus for measuring a diameter of a single crystal described under (2), as is particularly described with reference to FIG. 1 hereinafter.
(1) A method for measuring a diameter of a single crystal comprises the steps of causing the center of oscillation of a single crystal, which is grown according to the Czochralski method while oscillating, and a scanning line of a one-dimensional CCD camera to coincide with each other, and deciding a diameter of the single crystal from all images measured by use of the one-dimensional CCD camera over a measuring time which is longer than an oscillation cycle of the single crystal.
More particularly, for the measurement of the diameter of a single crystal, a growth portion of a single crystal 6, which is pulled up from a melt 3 of a crystalline starting material, is scanned on a plurality of scanning lines while moving the one-dimensional CCD camera 11 in parallel to the horizontal directions relative to the melt surface, and a diameter of the single crystal is obtained based on image signals from the one-dimensional CCD camera. The central position of a scanning line within a range of not smaller than a predetermined ratio (%) to the maximum value of the thus obtained diameter, is taken as a scanning position. Next, the growth portion is scanned, by means of the one-dimensional CCD camera 11, over a time longer than an oscillation cycle of the single crystal plural times at scanning intervals, each of which is shorter than the oscillation cycle of the single crystal. The diameters of the single crystal are calculated based on the image signals from the one-dimensional CCD camera, and a plurality of the diameters are selected from the resultant values in the order of decreasing value. In the method for measuring the diameter of a single crystal according to the invention, the average value of the selected diameters is determined as a diameter of the single crystal.
(2) An apparatus for measuring a diameter of a single crystal 6 to be grown from a melt 3 of a crystalline starting material according to the Czochralski method comprises a one-dimensional CCD camera 11 disposed at an outside of a chamber 7 to scan a growth portion of a single crystal via a window 8 provided at an upper portion of the chamber, an XY table 12 capable of moving the one-dimensional CCD camera in horizontal and vertical directions with respect to a melt surface, means for deciding a scanning position, at which an image of a growth portion of a single crystal is measured, from image signals of the one-dimensional CCD camera moved in parallel to the horizontal directions relative to the melt surface by means of the XY table, and means for determining a diameter of the single crystal based on image signals of the one-dimensional CCD camera, with which the single crystal is scanned at the scanning position plural times at Scanning intervals shorter than an oscillation cycle of the single crystal over a time longer than the oscillation cycle of the single crystal.
The term xe2x80x9cdiameter of single crystalxe2x80x9d used under (1) and (2) above means a diameter of a single crystal growth portion as scanned with the one-dimensional CCD camera, irrespective of a necking portion, a shoulder portion and a body portion of the single crystal.